Control circuit for radio receivers



1950 R. c. FERRAR 2,531,416

CONTROL CIRCUIT FOR RADIO RECEIVERS Filed Sept. 27, 1946' LIMITER IN V EN TOR. zioier/ IQ @11 2 ATTORNEY Patented Nov. 28, 1950 I TED S TATES TENT O FFl-CE CONTROL CIRCUIT FOR RADIO RECEIVERS Robert C. Fer-rar,'New Providence, N. .21.,assignor to Federal Telephone and .Radio Corporation, New York, N. Y., ,a corporation of Delaware Application September 27, 1946,'Serial N 0. "699,683

more outlying stations; and in these systems it has been a common practice to provide a common frequency channel for transmitting to all of the stations. Such a system has been used for example, in police communications and in other similar services to call a desired one of a number of remote mobile or field stations from the central control or calling station. In such a communication system it is desired to provide some means for calling and selecting the particular one of the call stations to which it is desired to transmit signals and to put this particular station into operating relation with the calling station.

It is the purpose of my present invention to provide a simplified and relatively economical system for accomplishing the calling operation and putting the called station into operating relation with the callin station. In accord ,ance with my invention, I utilize a single vibrating reed set. To enable the reed to operate at proper times, I provide a first control including relay means responsive to a received carrier signal, and this relay. when operated by the carrier, functions to render operative a irc it element such as an audio-amplifier to receive and amplify the modulation si nals. I connect at th ou pu of this circuit elemen a re ponsive d vic such as a rel y coil o perate the vibratory reed whenever a particular modulation signal is received at the particular frequency at which the vibratory reed is resonant.

When the reed is thus vibrated, it functions to render the output of the receiver operative and also to ren er opera iv another :controliw eluding relay means provided wi h a loclsein circuit with th nrs -menticn d relay m ans'to maintain the system in peration reg rdle s of whether .the calling modulation frequency is pre ent or not. In this wayr' he re eiv r is held operative to receive signals from the calling sta tion so long as carrier is present; but as soon as the carrier is discontinued, the relays are dropped back to their inoperative positions so that the receiver is again rendered. ready to receive a subsequent call.

2 The foregoing and other 'features of "my invention will be better understood from the following detailed description and the accompanying drawing which shows a portion of a receiver provided with means for rendering it operative by transmission of a call signal, in accordance with my'inventlon.

The receiver illustrated in "the drawing is of the frequency modulated type and comprises tuned intermediate frequency transformer I, having .a primary winding 12 and a secondary winding 3, the primary winding "being tuned by a condenser 4 and the secondary 'winding Lbeing tuned by a condenser 5, according :to a :common practice. The :primary :circuit of the transformer has impressed thereon the modulated intermediate frequencysignal over leads -'6 and fl; and these intermediate frequency signals may be supplied from the output of the intermediate frequency amplifier .of the :radio :receiver according to a well knowniconventional manner which need not bedescribedihere.

There is connected to :the secondary circuit of the transformer I a :limrter tube :8 Which may be a conventional type of limiter used in frequency modulation receivers, having a cathode 19,2. controlgrid It! and :an anode H. The output of :limiter tube 8 'is preferably, although not necessarily, aconnected to "a second limiter stage represented by the :diagrammatic rectangle =12, the output of which is connected to a discriminator stage 13 which may be some suitable form of discriminator well known in the art, which tunctions to deliver the modulation frequencies.

There is connected to the output of the discriminator an audio amplifier stage comprising a vacuum tube I 4 which "is shown to be of the triode type :having -,a filamentary cathode I5, control grid l5 and anode H. An audio frequency output transformer i8 is connected to the output of the audio amplifier l4 and asuitable loudspeaker or receiver 19 connected to the output of the output transformer.

iI-t is desired to maintain this receiver inoperative until a calling station transmits a call of a particular character calculated to render the receiver operative, and according to my invention, I have provided in association with the receiver, means for doing this. I have provided means for calling and rendering the receiver operative upon receipt of a call, and holding it operative so long as a carrier frequency is received from the calling station. This control means co prises a trigger system, and the particular triggering means I have provided is a a filled thyratron tube 20 which is shown as a type having a cathode 2!, a control grid 22, a shield grid 23 and an anode 24. There is connected in the anode circuit of the tube 20 a relay coil 25 in series between the anode 24 of a source of D.-C. operating voltage marked B+, the negative side of this D.-C, voltage source, marked B-, being connected to the cathode 2| through the tappedof? portion of a potentiometer 2 the opposite end of the potentiometer being connected to the shield grid 23. There is connected between the anode 24 and the shield grid 23 a condenser 2'1. The value of the anode voltage supplied from the B+ voltage source is normally sufficiently high to cause relaxation type oscillations in the circuit of condenser 21 and coil 25, the frequency of these oscillations being determined by the particular values of this coil and condenser.

Provision is made for stopping the oscillations when a carrier signal is received from the transmittin station; and this means comprises a rectifyin circuit comprising a resistor '28 and a condenser 29 in parallel with it, the parallelarranged resistance and condenser being connected between the cathode 9 and the lower end of the secondary resonant circuit of transformer I. By reason of this resistor-condenser arrangement, rectification occurs in the grid circuit of the limiter tube 8, in the presence of carrier signals, with a resulting unidirectional current flowing through resistor 28. One side of resistor 28 is connected to ground, and the other side is connected through a resistor 30 to the grid 22 of the thyratron. Since the cathode 2! of the thyratron is also connected through to ground (through potentiometer 26 and two resistors 3! and 32 all in series), the eifect of the unidirectional current through resistor 28 is to make grid 22 more negative. When the grid 22 is thus made negative, it blocks the thyratron Stopping the oscillations in the circuit of relay coil 25 the complex impedance of which is principally resistive, and capacity 21, and accordingly shutting down the current flow through relay coil 25.

The relay controls three armatures 33, 34 and 35, these armatures being adapted to make contact with the respective armature contacts 36, 31 and 38. The arrangement is such that when the circuit of condenser 2'! and coil 25 is in its oscillating condition, there is enough current through the coil to hold all three of the armatures down and away from their respective contacts; but when the oscillations are stopped by the presence of a carrier wave at the receiver, the three armatures drop back against their contacts.

When armature 34 closes against its contact 31, it closes the previously open circuit through the filament l of audio-amplifier H! from across the source of filament voltage, the posi-- tive and negative terminals of which are marked +f and f, respectively. This puts the previously inoperative tube is into operating condition.

Armature 33, when closed against its contact 36, functions to place a short circuit across the bias bleeder resistor 32. This shorting of the bleeder resistor coincident with the placing into operation of the previously inoperative audioamplifier is in accordance with gOOd practice in those cases where the power supply is so arranged that the added drain on the power supply from the audio-amplifier would otherwise produce undesirable changes in the delivered power voltages. It i desirable to compensate for the change in power supply voltage due to the added drain, by correspondingly changing the resistance in the grid biasing circuit.

The other armature 35, when closed against its contact 38, functions to set up a lock-up circuit through an armature 39 and contact ll] of a relay coil 4!, when this latter relay becomes energized, as will be described hereinbelow.

Even though the normally inoperative audio amplifier M is rendered operative by the presence in the receiver of a carrier frequency, as has just been described, this alone will still not render the receiver operative to deliver signals to the loudspeaker or headphones or the like l9, because there still exists a short circuit across the primary winding of output transformer 18. This short circuit exists over conductors 42 and 43 and the normally closed contact 4 with armature 45. Armature 45 is one of two armatures controlled by relay coil ll, the other armature being the previously mentioned armature 39. So long as relay coil All remains in its deenergized condition, the two armatures 39 and 35 are in their up position, that is, armature 39 remains out of contact with its contact 40, and armature 45 remains in contact with its back contact M and out of contact with its front contact 46.

In order to complete the placing into operation of the receiver so that signals can be heard through the loudspeaker or the headphones I9, I provide an electromagnetic coil 41 in the anode circuit of amplifier tube It, in series between the anode H and the primary winding of the output transformer l8; and I provide in conjunction with the cell a vibratory reed 53 sharply resonant at a definite frequency so that when this resonant frequency, or one very close to it, is sent through the coil 41, the reed will vibrate. The reed has connected with it a pair of contacts 49 and 50 arranged to vibrate in and out of contact with each other by the vibration of the reed. The vibrating of these contacts together serves to send current intermittently through relay coil 4| from the B+ source of high D.-C. voltage, the circuit for this operation passing from ground through the relay coil 4|, over conductor 5|, through the contacts 50 and 49, and conductors 52 and 42 to the B+ terminal of the voltage supply. Condenser 53 is used to integrate the pulses of direct current resulting from intermittent closure of contacts E9 and 50 in order to provide a reasonably steady flow of current through relay coil 4 I.

As soon as coil M becomes energized by the vibrating of the reeds, the armatures 39 and 45 are pulled in, armature 39 making contact with its contact 40 and armature 45 breaking away from its contact 44 and making contact with 66. The breaking of contact at 44 serves to remove the short circuit across the input winding of transformer l8 so that the receiver now becomes fully operative to deliver signals to loudspeaker [9. The making of contact at armature 39 and contact 40 serves to complete the lock-up circuit which has been set up over armature 35 and'its contact 38, and is now completed through relay winding 4| from the B+ source of D.-C. voltage to ground. Accordingly, the current from the voltage source and through coil M will now pass over armature 39 and contact ii! to maintain coil 6! energized even after the reed 48 stops vibrating; and this energization of coil 4| with consequent operativeness of the receiver will continue so long: as carrier signal is present at the receiver to hold armature 35 against'its contact 38. Accordingly, the ability of the receiver' to: deliver signals from the transmitting station will thereafter'depend on. the presence of carrier, and'will not depend on the modulation signals which are present.

The resonant frequency to which the vibrating reed 48 is made resonant is not especially critical, but preferably itwill be in the low audio-frequency range and the resonance will be quite sharp. Accordingly, the reed will be vibrated: by the. presence of a modulation signal of this particular resonance frequency, which will bethe.

call frequency of the station.

In operation, the operator at the transmitting or calling station will turn on his carrier which will be picked up at the receiving station, and the carrier will produce the rectified current in re sistor 28 to deenergize relay coil and render the audio-amplifier l4 operative. Then, to call and put the receiving station finally into operation, the operator at the transmitting station will transmit the call. frequency for a short time by modulating his transmitted carrier by the particular call frequency, which has been designated, so that the transmitting station will send out a carrier frequency which is frequency modulated by the call si nal. This call signal will be separated at the discriminator A3 of the receiver in the usual manner of frequency modulated receivers, and will be delivered as a call signal to the grid I5 oi amplifier l4, and accordingly sent through the coil 4'! at the output of the audio amplifier to operate the reed, which in turn renders the receiver fully operative to transmit the signals to the speaker 19 and to hold the receiver operativ by operation of the relay coils 4| and 25 as has been explained hereinabove.

The particular values of the various circuit elements which have been used is not especially critical and it is only required that proper interrelated values be given to the several coils, condensers and resistors to make them mutually cooperate according to good practice to produce the efiects described. For the thyratron tube 20, a tube of the 2-D-2l type has been found satisfactory. Values of other circuit elements which have been found satisfactory for use with this type of tube are as follows:

Condenser 2], 0.1 microfarad Relay coil 25, 12,000 ohms Potentiometer 26, 200 ohms Resistor 3 l, 200 ohms Resistor 32, 200 ohms Resistor 28, 22,000 ohms Condenser 29, .00005 microfarad Resistor 30, 220,000 ohms Tube Ill-type 2E30: Filament voltage for tube 14,

6.3 volts 7 Plus B voltage, 180 volts In operating the foregoing thyratron type tube in the circuit, it has been found that oscillations can be made to cease with a change in bias in the order of 0.4 volt. The absolute value of bias about which the 0.4 volt change is operated is adjustable by means of the potentiometer 2E, and in practice this potentiometer has been made adjustable from approximately 0 to 8 volts. This is made adjustable in order to allow for variations in noise level which are to be expected in various localities of the receiver installations.

Although a particular form of receiver and operating circuit has been illustrated and described in detail as showing a particular embodimentof my invention, it will be understood that variations and modifications may suggest themselves in particular instances, all within the scope of the invention. The invention, moreover, is not limited to use with frequency modulation communication systems, as it is also applicable to other kinds of modulation systems such as am?- plitude modulation systems.

Among the various modifications of the foregoing described system which may be made Within the scope of'the invention is a carrier squelch operation. This may be done by simply short 'cirsuiting the contacts 39 and 40 of relay coil 4|. If no squelchat all is desired, the contacts 39 and 40 would be shorted and the filament circuitof thyratron 2G opened.

It will be recognized that by my invention I have provided a highl useful calling and operating system for communicating from a transmit ting station to an outlying receiving station,- for example, a mobile field unit, such as a police car or the like which may be called from headquarters. The system is singularly simple and rela-' tively inexpensive in comparison with other kinds of systems which have been proposed for similarpurposes.

I claim:

1. In a system for rendering a signal receiver operative in response to a predetermined. signal, means to detect a signal modulated carrier, a re producing device, a predetermined signal responsive device, means to apply received signals to both said devices, control means to normally render said reproducing device inoperative, said signal responsive device being operative in response to said predetermined signal to operate said control means to render said reproducing device operative, and means responsive to reception of said carrier to maintain said control means in said operative condition, said control means having means to render said signal responsive device inoperative in response to said control means being in said operative condition.

2. In a system for rendering a signal receiver operative in response to a predetermined signal, means to detect a signal modulated carrier, a reproducing device, a predetermined signal responsive device, means to apply received signals to both said devices, control means to normally render said reproducing device inoperative, said signal responsive device being operative in response to said predetermined signal to render said control means operative, to render said reproducing device operative, said control means having means to hold it in operative condition when actuated and means to render said signal responsive device inoperative in response to said control means being in said operative condition.

3. In a system for calling a signal receiving station from a signal modulated carrier transmission station, the receiving station having a normally inoperative vacuum tube stage, a normally inoperative signal reproducer and a vibratory element resonant at a predetermined signal frequency coupled to the output of said vacuum tube stage, a gaseous discharge tube having in circuit therewith an oscillatory system which is in a nor mally oscillating condition, means responsive to received carrier for stopping the oscillations in said system, means responsive to said oscillatory system for rendering said normally inoperative vacuum tube stage operative upon stopping of the oscillations, said vibratory element responsive to the receipt of a modulation signal at its resonant frequency from the output of said vacuum tube stage for vibrating, and a circuit controlled by the vibrations of said vibratory element for rendering said signal reproducer operative and a second means for holding said vacuum tube stage and signal reproducer operativ in response to the reception of solely the carrier signal after the modulation signal has ceased.

4. In a system according to claim 3 in which the gaseous discharge tube is a thyratron.

5. In a system according to claim 3 in which the gaseous discharge tube is a thyratron, and the means responsive to the oscillator system comprises a relay which renders said vacuum tube stage operative.

6. In a system for calling a signal receiving station from a signal modulated carrier transmission station, the receiving station having a normally inoperative vacuum tube stage and a normally inoperative audio output; a vibratory element resonant at a predetermined frequency of the calling signal, a thyratron having an oscillatory output circuit normally maintained in an oscillating condition, rectifying means for developing aunidirectional voltage responsive to-received carrier, means for applying said unidirectional voltage ona control electrode of the thyratron in the polarity which stops the oscillations, a first relay operable by the stopping of the oscillations, a connection from the relay to the vacuum tube stage for rendering the stage operative upon operation of the relay, said vibratory element being responsive to the output of said stage, a second rela energized in response to the vibrating of the vibratory element, said audio output being rendered operable in response to the energization of the second relay, and a lock-up circuit through contacts of the second relay closed by operation of second relay and through contacts of the first relay closed by by operation of said first relay for holding the second relay energized in response to the first relay being maintained operable by continued receipt of the carrier.

ROBERT C. FERRAR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,635,779 Carter July 12, 1927 2,202,432 Santucci May 23, 1940 2,264,018 Case Nov. 25, 1941 2,361,653 Van Roberts Oct. 31, 1944 OTHER REFERENCES Millman and Seely, Electronics, McGraw Hill (1941), page 334. (Copy in Library of Congress.) 

